Thursday, April 28, 2011

Weekend Observing Challenge: A Gathering of Galaxies - Hickson 44

Written by Tammy Plotner

If you turn your eyes towards Leo, you just might discover a group of galaxies which reside close to our own Milky Way - Hickson Compact Group 44. At only 60 million light years away, this diverse and interesting collection has quite a story to tell!

Some three decades ago, Canadian astronomer, Paul Hickson set about the task of completing a list of 100 galaxies clusters. But, they couldn't be just any set of galaxies - they had to be isolated, compact and within a limited magnitude range. His purpose was to study them for unusual redshifts among their members - and to improve our knowledge of galactic evolution. From his work came the theory that perhaps all well-known galaxies once emerged from such clusters and this hypothesis also contributed mightily to our understanding of dark matter as well. What Hickson left us with is a legacy of beautiful objects that challenge not only the telescope - but the mind as well.

In this photo done by Warren Keller, you will see from 11 o'clock: NGC 3193; 3190 and 3187 at center, and 3185 at 6 o'clock. A closer look reveals two tiny galaxies PGC (Principal Galaxies Catalog) 2806871 near 8 o'clock and PGC 86788 near 5 o'clock. As you can see, this group is an interesting collection of galaxy types - from barred spiral to elliptical in structure... close enough to share material as they gravitationally interact.

While some of you may recognize the three principle players in this galactic act as the "Leo Trio", take a closer look at barred spiral NGC 3190. It was first discovered by Sir William Herschel in 1784 and was home to two supernova events in 2002. Lurking at its heart is an active galactic nucleus (AGN), home to a super-massive black hole. While the discovery of the rare Type Ia supernova was unusual enough, adding a second similar supernova event occurring simultaneously made this galactic action even more rare. Two young x-ray emitting events, set against a record breaking amount of obscuring dust!

Perhaps the interaction with nearby NGC 3187 is the root cause? It is, after all, evolving. Studies indicate an evolutionary sequence for Hickson compact groups in which the amount of diffuse light increases with the dynamical evolution of the group. "Compact groups are associations of a few galaxies in which the environment plays an important role in galaxy evolution." says J. A. L. Aguerri (et al). "The low group velocity dispersion favors tidal interactions and mergers, which may bring stars from galaxies to the diffuse intragroup light. Numerical simulations of galaxy clusters in hierarchical cosmologies show that the amount of the diffuse light increases with the dynamical evolution of the cluster."

While this group of galaxies is evolving and interacting together across vast distances, you can collect them all in the same eyepiece view about halfway between Gamma and Zeta Leonis (RA: 10h18m00.4s Dec: +21°48'44"). They are by no means easy, the faintest of which is magnitude 13, but it can be accomplished with a minimum of a 150mm telescope under dark, clear skies. Remember, the beauty is in the challenge... and the discovery!

Be sure to check out Warren Keller's Image Processing for Astrophotography (IP4AP) software!

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